Cryptology (or cryptography) is about communication in the presence of adversaries or potential adversaries. In medieval times diplomats had to communicate with their superiors using a messenger. Messengers could be killed, and letters could be captured and read by adversaries. During times of war, orders from military headquarters needed to be sent to the line officers without being intercepted and understood by the enemy. The case of a war is an extreme example where the adversary is clearly defined. But there are also situations where the existence of an adversary is less obvious. For example, corporate deals and all negotiations must remain secret until completed. Sometimes two parties want to communicate privately even if they do not have any adversaries. For example, they wish to exchange love letters, and confidentiality of messages for them remains a very high priority. Thus, a classic goal of cryptography is privacy. Authentication is another goal of cryptography which is any process by which you verify that someone is indeed who they claim they are. We use passwords to ensure that only certain people have access to certain resources (for example, if you do your banking on the Internet you do not want other people to know your financial situation or to tamper with your accounts). Digital signatures are a special technique for achieving authentication.